Power plant generator circuit breaker? 8

[Marmite]

In a power plant where the station auxiliaries are fed from 2x 132/11kV transformers straight onto the station main 11kV switchboard, and the generator is separately connected to a 11/132kV delta star step up transformer feeding straight to a 132kV C/B on the 132kV bar at the main substation with no other 11kV connection and no unit transformer, would it be normal to provide a generator circuit breaker at 11kV? The consultants are suggesting that an 11kV generator circuit breaker is required, and one of the reasons cited is to earth the LV winding of the 132/11kV generator transformer for work on the transformer. It's an expensive way of providing an earthing facility, and personally I can't see why it would be required. Is the generator circuit breaker normally installed in this kind of scenario?
The generator will be connected to the generator transformer using 11kV phase segregated bus ducting.
I'd be grateful for any comments.
Regards
Marmite

 

[davidbeach]

No, save the cost and trouble of the 11kV breaker. It won't provide any benefits.

 

[rcw retired EE]

A generator breaker can minimize damage to the GSU transformer from a transformer internal fault. A unit connected generator cannot be de-energized quickly after a fault. The excitaiton and turbine can be tripped but it still takes time for the voltage to decay. The residual energy fed to the transformer as the turbine-generator coasts down is enough to escalate a simple rewind fault repair into a trasnformer core replacement.

If this is a new installation, cost savings could be realized by suppling the auxiliary loads direct from the generator 11 kV bus, saving the 132 kV/11 kV transformers and two 132 kV breakers.

A generator breaker is also a convenient place to connect VT's, CT's, disconenct switches and grounding switches. A generator breaker makes it easier to synchronize since it is done at the low voltage.

 

[Marmite]

Thanks guys.
Regards
Marmite

 

[slavag]

Good answer of RCWilson.
star from me too.

 

[prc]

I doubt whether GCB will be economically justifiable at 132 kV.At 220 kV &above GCB can give advantages-main one is the economy as the station start up transformer and the bay with switchgear can be avoided as cold start can be performed using unit transformer feeding to unit auxiliary transformer with GCB open.In addition to simplified operational procedure, improved protection for generator and unit transformer, with better selectivity, is another advantage with GCB system.

Limited damage to transformer and generator during internal faults is another advantage claimed for GCB.Let me bring up my reservations.GCB will clear fault within 4 cycles.Without GCB, power will flow from generator to fault till generator de-excitation ends( The generator rotor will continue to rotate due to the momentum with reducing rpm)Typical de-excitaion times are 8 sec with static rectifiers,10 sec with rotating DC shaft machines,20sec with rotating diodes.But it is to be remembered that fault current from generator will be limited by the generator impedance plus unit transformer impedance.This may be more than 30%, limiting fault current to 2-3 times full load current.So eventhough the faulttime may be more,but current magnitude will be less thereby limiting the energy release. Many times energy release is found much more when fault is at HV line end of transformer- ie breakdown of lower part of HV bushing, creep flash over from HV line end of winding to neutral.The fault current,fed from grid, on such occasions( cleared by HV CB ) is found above 10 KA at 220 /400 kV causing much more damage inside the transformer.

Core damage in transformer will happen only when LV winding collapses on to core causing a power current flow though core.Such current will scatter in to core and escape out through core earthing lead.Current flow will result in numerous scattered small holes in core laminations. But such failures are very rare in unit transformers, but more in small and medium sized transformers, failed by flow of short circuit current through them.

When GCB is used, insist for a surge absorber and LA on the side connected to LV side of unit transformer. This is to take care of the transferred surge in to LV winding of transforer when GCB is in open position.